This invention relates to a sawtooth generator comprising a current source, capacitive means for integrating a current to be supplied by the current source, and an electronic switch coupled to the current source and to the capacitive means, which switch has a gate input for the supply of a gate signal for switching the sawtooth generator to a first operational state in which the sawtooth generator supplies a sawtooth-shaped signal at its output and to a second operational state in which the capacitive means are included in a control loop. The capacitive means is at least partly discharged by discharging means and the output is controlled to a DC voltage value. The control loop comprises a comparison means with a first input which is coupled to the output of the sawtooth generator and with a second input for the supply of a reference signal, which comparison means supplies a control signal for controlling the DC voltage value.
The invention also relates to an oscilloscope provided with such a sawtooth generator.
A sawtooth generator of the kind mentioned in the opening paragraph is used, inter alia, in a horizontal deflection circuit of an oscilloscope provided with a cathode ray tube as a display medium for showing the waveforms to be measured. The signal to be measured is in that case applied to a vertical deflection circuit and to a trigger circuit in a horizontal deflection circuit which starts the sawtooth generator periodically. The sawtooth generator supplies a signal for the horizontal deflection of a cathode ray in the cathode ray tube. The horizontal deflection system forms a so-called time base. Measuring instruments other than oscilloscopes, for example a counter, may also comprise such a time base.
Such a sawtooth generator is known from British Patent no. 1,516,200. In a first operational state of the sawtooth generator, a sawtooth voltage is formed at the output of this generator through integration of a current passed by a capacitor. In a second operational state, the capacitor is discharged and the voltage at the output of the sawtooth generator is controlled so as to achieve a constant value, for example 0 V, by means of a control loop. Although the control loop described in the cited British Patent no. 1,516,200 gives improved results compared with a non-controlled output voltage in the second operational state as regards transitional effects, the sawtooth generator disclosed nevertheless leaves something to be desired. The control loop, which is switched on via an electronic switch, comprises a diode whose anode is connected to an output of a comparison means and whose cathode is connected to the capacitor. Upon switching over from the second control state to the first operational state in which a sawtooth voltage is generated across the capacitor, the anode voltage will suddenly drop, as a result of which the diode will be blocked. The diffusion charge present in the diode will give rise to an undesirable transitional effect between the second and the first operational state. Overshoot to below the controlled DC voltage value will then take place before integration is carried out in a correct manner. Furthermore, different integration capacitors will be switched off when such a sawtooth generator is used in an oscilloscope with various ranges, such as decades. This gives rise to differences in control behavior of the control loop in the second operational state. A comparatively small capacitance value means a comparatively high loop amplification with the risk of an unstable control characteristic, whereas a comparatively high capacitance value means a comparatively low loop amplification so that the control loop could become too slow.